This invention relates to a noise eliminating circuit for an AM receiver that eliminates noise pulse components contained in an amplitude modulated wave, and more particularly, it relates to a noise eliminating circuit which opens the path of an RF stage or IF stage at the incoming of a noise pulse to thereby eliminate such noise, while at the same time, compensating for the carrier component of the incoming signal that is absent in the RF or IF stage output during switching by producing an oscillation frequency which is the same as the carrier frequency immediately before the switching.
If a noise pulse component contained in an amplitude modulated wave is eliminated, for example, by switching to open the path of an IF stage, during the period when such noise component exists, the IF signal is zero during this period, and so, the carrier component of the IF signal must be compensated as long as the IF stage is open. Therefore, in the conventional noise eliminating circuit of this type, an oscillator circuit has been used which is oscillated during the switching period to produce a fixed frequency for compensating for the absence of the carrier component.
However, in such a conventional of noise eliminating circuit, the oscillation frequency is fixed, and so, if the IF signal frequency differs from the oscillation frequency, a frequency difference appears at the termination of the switching period so as to generate another noise.
According to the conventional technique, when an audio signal exists and if the main signal path is switched to open to eliminate a noise pulse component as described, the cessation of the audio signals frequently causes a reversing phenomenon, a phenomenon where the noise sounds louder than before switching. No reversing phenomenon occurs if a relatively large noise pulse component is contained in the audio signal, but if the noise component is relatively small, a great distortion is developed in the audio signal after switching to eliminate the noise. It is therefore desired to effect said noise-eliminating switching only for eliminating a noise of such a high level that it causes no reversing phenomenon. However, in the case where an audio signal exists, a relatively small noise component is not a serious matter to worry about as long as the reversing phenomenon can be avoided, but if no audio signal exists, even such a relatively small noise component sounds too distinctively to be disregarded because it is not masked by the audio signal. As a consequence, it is difficult to prevent the phenomenon to a satisfactory extent.
It is therefore one object of this invention to provide a noise eliminating circuit that eliminates a noise pulse component by switching to open an RF or IF stage, and prevents generation of another noise by compensating for the frequency of the carrier of the incoming signal that has been absent during the switching period with a frequency so controlled as to be equal to said carrier frequency.
It is another object of this invention to provide a noise detecting circuit which detects, when an audio signal exists, only a noise pulse noise of a high level that causes no reversing phenomenon, and when no audio signal exists, detects even a noise of an relatively low level, so as to generate a pulsed signal that is capable of triggering a switching circuit for eliminating the noise component.